Mixed (Ti,Zr)C offers significantly higher hardness compared to its monocarbide constituents owing to solid solution hardening. However, the (Ti,Zr)C system has a miscibility gap in which the carbide can decompose into TiC-rich and ZrC-rich phases at high temperature. This limits the utilization of (Ti,Zr)C, where structural stability at high temperatures is sought, but we here show, using in-situ neutron diffraction during aging at 1600 °C, how small additions of other carbides can significantly retard the decomposition. The effect on decomposition kinetics by addition of 1 mol% HfC or NbC is shown and thermodynamics calculations and scanning transmission electron microscopy experiments aid to propose that the altered decomposition kinetics can stem from a narrower miscibility gap and altered interface chemistry. Although the decomposition morphology suggests that phase separation proceeds through discontinuous precipitation, we find that only the TiC-rich decomposition product nucleates with a composition far from equilibrium and evolves with time.